Since aromatic polyester resins typified by polyethylene terephthalate and polybutylene terephthalate have excellent mechanical properties, electrical properties, heat resistance, weatherability, water resistance, chemical resistance, solvent resistance and processability, they are used as engineering plastics in a wide variety of fields such as automobile parts and electric and electronic parts.
Meanwhile, in the automobile field, for the purpose of improving fuel efficiency as one of environmental measures, it is desired to reduce the weights of parts to be mounted on vehicles, and resin parts are becoming thinner and lighter. Importance is now attached to the flowability of a material so that a thin part can be manufactured therefrom. With respect to the quality of each part, the further improvement of mechanical strength such as tensile strength and moist heat stability such as hydrolysis resistance is strongly desired.
In recently years, the aromatic polyester resins have often been used in sheets and films. Especially for photovoltaic power generation (solar cells) application, when an aromatic polyester resin is used as a back sealing film (back sheet film), weatherability and hydrolysis resistance, especially long-term hydrolysis resistance, are required.
Since the aromatic polyester resins are inferior in durability to fluorine-based resins and polyethylene-based resins, various methods for improving durability are proposed.
For example, Patent Documents 1 and 2 disclose that hydrolysis resistance is improved by adding a polycarbodiimide to an aromatic polyester resin.
However, it is known that when a polycarbodiimide compound is used as an end capping agent for a polymer compound, the viscosity of the aromatic polyester resin is greatly increased by a crosslinking reaction with the polyester
(Patent Document 3)
To mold this polyester whose viscosity has been increased, it is necessary to raise the molding temperature. However, when the molding temperature is simply raised, the obtained molded article becomes yellowish, or molding itself become difficult because of the too high temperature should be set.
When a carbodiimide compound is used as an end capping agent for a polymer compound, a compound having an isocyanate group is liberated along with a reaction that the polycarbodiimide compound is bonded to the terminal of the polyester, thereby generating a characteristic to the isocyanate compound odor worsen the work environment.
To solve this problem, there is proposed a resin composition comprising a novel compound having carbodiimide rings, each having only one carbodiimide group (may be referred to as “cyclic carbodiimide compound” hereinafter) as a carbodiimide having a structure that an isocyanate compound is not liberated (Patent Document 4). In this proposal, a resin composition which does not liberate an isocyanate and has a high level of hydrolysis resistance is obtained.
However, when a cyclic carbodiimide compound having at least two carbodiimide rings, each having only one carbodiimide group, is selected from among the above proposed compounds as a thermoplastic aromatic polyester, there arises a new problem that melt viscosity increases as compared with a case where a conventionally known polycarbodiimide compound is used though the degree of improving hydrolysis resistance is high.    (Patent Document 1) JP-A 8-73719    (Patent Document 2) WO2010/018662    (Patent Document 3) Japanese Patent No. 3618940    (Patent Document 4) WO2010/071213